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We can arrange activities into a hierarchy and feed activities to the workflow engine as instructions to execute. The activities can direct workflows involving both software and humans. All activities in WF derive from an Activity base class. The Activity class defines operations common to all activities in a workflow, like Execute and Cancel. The class also defines common properties, like Name and Parent , as well as common events like Executing and Closed the Closed event fires when an Activity is finished executing.
The screenshot below shows the Activity class in the Visual Studio class designer:. WF ships with a set of ready-made activities in the base activity library. The base activity library also includes activities to wait for events, to invoke web services, to execute a rules engine, and more. For instance, pizza delivery workflows could benefit from custom activities like SendOrderToKitchen or NotifyCustomer. All custom activities will also ultimately derive from the base Activity class.
The workflow engine makes no special distinction between activities written by Microsoft and custom activities written by third parties. For instance, a SendOrderToKitchen custom activity could encapsulate a web service call and other processing logic inside. This activity is obviously specific to the restaurant problem domain. A developer will be more productive working with this higher-level abstraction than with the primitive activities in the base activity library.
NET, and XML are general-purpose languages and have the ability to solve a wide array of different problems. We can use C to develop solutions for pizza restaurants as well as hospitals, and the language works equally well in either domain.
A domain-specific language excels at solving problems in a particular area. A domain-specific language for restaurant workflow would boost productivity when writing software for a restaurant, but would not be as effective when writing software for a hospital. These extensions plug into Visual Studio to provide a number of features, including a visual designer for constructing workflows.
A screenshot of the visual designer is shown on the next page. The Toolbox window will list the activities available to drag onto the design surface. We can add our own custom activities to the Toolbox. Once an activity is on the design surface, the Properties window will list the activity's properties that we can configure, and the events we can handle.
The Toolbox window is shown below:. Here is the XAML generated by the designer for the workflow we saw earlier:. Our workflow is trivial and contains only a single activity inside—a CodeActivity. When the workflow engine executes the CodeActivity , the CodeActivity will invoke a method specified by the ExecuteCode attribute. In WPF, XAML declaratively constructs a rich user interface consisting of not only buttons and labels, but also animation storyboards and data templates. Partial classes, introduced in.
NET 2. We can add members to the generated class by defining a class with the same name and with the partial keyword. The designer will flag an activity with a red exclamation point if the activity raises validation errors. For example, a CodeActivity will display a red exclamation point until we set the ExecuteCode property.
Without a method to invoke, the CodeActivity is useless, but the validation catches this problem early and provides visual feedback. The designer also provides debugging features. We can set breakpoints on an activity in the workflow designer. When execution stops, we can look at the Call Stack window to see the activities previously executed in the workflow instance.
The debugger commands Step In, Step Out , and Step Over all work intuitively; for instance, the Step In command will move to the first activity inside a composite activity, while Step Over executes the entire composite activity and moves to the next sibling. We can even specify color and border styles for specific activity types. Through Visual Studio, we can create new themes, or modify existing themes. All this styling ability isn't just to make the designer look pretty in Visual Studio, however.
The WF designer is a component we can host inside our own applications. First, we can host the designer and allow the non-developer types a. By providing custom themes, we can match the designer look with the look of our application.
In Windows Workflow, the processor is in the WF runtime. To start a workflow party, we first need a host for the runtime and workflow services. Like ASP. Like the ASP. NET runtime, WF needs a host process to load, initialize, and start its runtime before anything interesting can happen. Unlike the traditional server-side usage of ASP. NET, however, WF will be useful in a variety of different hosts.
We can host WF in a smart client application, a console application, or a Windows service, for instance. The class diagram in the screenshot below features the primary classes we use to execute workflows in WF. Creating an instance of the WorkflowRuntime class and calling StartRuntime is all we need to spin up the workflow execution environment.
WorkflowRuntime defines methods that allow customization of the execution environment. The class also defines events we can listen for during execution. The CreateWorkflow method returns an object of type WorkflowInstance. The WorkflowInstance class represents an individual workflow. The Start method on the workflow instance object will begin the execution of a workflow. If an exception occurs, the workflow will invoke the Terminate method which leads to the runtime raising a WorkflowTerminated event.
A typical sequence of calls is shown in the screenshot next. The WorkflowRuntime and WorkflowInstance classes are arguably the most important classes needed at run time, but they are not the only classes available. The WorkflowRuntime class provides only the basic features for executing workflows. Don't worry, these features are available through an extensibility mechanism of WorkflowRuntime — the AddService method.
AddService allows us to make one or more services available to the runtime. These services might be custom services we've written specifically for our domain, like a custom scheduling service, or they might be services already written by Microsoft and included with WF. Let's continue our tour by looking at the services already available.
The DefaultWorkflowSchedulerService creates new threads to execute workflows. Because the threads are separate from the host application, the workflows do not block any application threads and execute asynchronously. The maximum number of simultaneously executing workflows is configurable. Server-side applications typically pull a thread from a pool to service each client request.
It makes sense to loan the thread to the WF runtime, and let the runtime execute the workflow synchronously on the existing request thread instead of using two threads per request, which could reduce scalability. The default transactional service is an instance of the DefaultWorkflowTransactionService class. Activities inside a running instance of a workflow, and the services operating on the same instance, can all share the same transaction context. WF relies on the implementation of transactions in.
NET's System. Transactions namespace. The Transaction class offers a lightweight, auto-enlisting, and promotable transaction. The transaction can start as a local transaction, and later the runtime can promote the transaction to a heavyweight, distributed transaction if needed.
Instead, the runtime can persist the state of the workflow to a durable store and unload the instance from memory. In 30 days or hopefully, less , the runtime can reload the workflow instance and resume processing. The WF runtime will automatically persist a workflow that is idle or suspended when a persistence service is present. Of course, we'll need a database schema that the persistence service understands.
A tracking service will tell the runtime the type of information it wants to know about workflows using a tracking profile. Once the service establishes a profile, the service can open a tracking channel to receive events and data. Chapter 5 includes more details on tracking profiles and channels. The runtime does not start a tracking service by default, but we can programmatically add a tracking service or configure a tracking service with an application configuration file for the runtime to use.
Maybe you've had one of those product managers who is always at your desk, asking "are you done, yet? The sample isn't meant to demonstrate all the capabilities of the platform, but give a general feel for creating and running a workflow with WF. Supported development tools for the. The extensions are not compatible with the Express editions of Visual Studio We'll choose C as our language and select the Sequential Workflow Console Application template see the screenshot on the next page.
The template gives us a project with references to all the correct WF assemblies, an empty workflow, and a Program. Right-click the workflow and select Delete so we can start a workflow from scratch. If we click to expand the node containing Workflow1. As we mentioned earlier, the partial class will combine with the class generated from the XAML to produce a single type. Let's modify the class in Workflow1. If we double-click the.
We can drag a While activity from the Toolbox and drop the activity between the start and end point of our workflow. The While Activity executes a child task until some condition is met. Our next step is to drag a Code activity from the Toolbox into the center of the While activity. At this point, our designer should resemble the following screenshot:. Notice both activities display a red exclamation point. The activities are failing their validation checks.
We can hover the mouse cursor over the exclamation points and open a smart tag to view the validation error. If we tried to compile the program we'd see these same validation errors as compilation errors. We'll fix these errors now. The Code activity requires us to assign an event handler for the ExecuteCode event. We can set the event by opening the Properties window F4 and clicking the Code activity to set focus.
Double-clicking in the empty space beside the ExecuteCode property will send us into the code-beside file and generate a new event handler.
We can place the following code into the event handler. This code will ask the user if a bug is fixed, and then read a key press. The Code activity should now pass validation, so we can turn our attention to the While activity.
A While activity requires a valid Condition property. Several activities in the base activity library work with conditions, including the IfElse, ConditionedActivityGroup , and Replicator activities.
These choices represent the two techniques available to express a condition, the first being with code a method that returns a Boolean value , the second being with a rule. Let's select the RuleConditionReference. A rule condition is a named expression that evaluates to true or false, and can live in an external. A plus sign appears beside the Condition property, and we can click the sign to expand the property editor.
When the Condition property expands, the Property window gives us the ability to set a ConditionName and an Expression. Clicking on the ellipsis … button in the Condition name will launch a Select Condition dialog box.
We want the While activity to loop until the bug is fixed. Our rule is! Once we've entered the condition notice the editor provides IntelliSense , we can click OK. We should select Condition1 and press OK. The While activity should now have a ConditionName and Expression set, and pass validation.
Now we need to open the Program. We need to host the WF runtime and ask the runtime to execute our workflow. The item template for a workflow project provides all the boilerplate code we need. Let's review the code:. The code wires up event handlers to the runtime so we know if a workflow terminates because of an exception , or completes successfully.
The code instantiates our bug-fixing workflow using the CreateWorkflow method, passing the type of our workflow. Search the Wayback Machine Search icon An illustration of a magnifying glass. Sign up for free Log in. EMBED for wordpress. Want more? Advanced embedding details, examples, and help!
The material is very well presented with code examples and explanations. Also, I love how the authors discuss the underlying architecture, enabling me to get a really deep understanding of the technology to efficiently design and build my own projects. Many tough problems traditionally faced by application authors, such as state management in the presence of long-running activities think weeks or months!
Dharma Shukla and Bob Schmidt present the workflow technology under and made accessible by the new workflow foundation in the. NET 3. NET Framework, and who wants to achieve declarative enlightenment. Don't miss out. Developers working on web services will want to learn about this technology from this book; it comes straight from the source and explains the technology well and in depth.
WF programs are assembled out of resumable program statements called activities, which provide encapsulation of both domainspecific logic and control flow patterns reflective of real-world processes. In Essential Windows Workflow Foundation, two.. Print version record. There are no reviews yet.
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